1. Field of the Invention
This invention relates generally to electroluminescence displays and more particularly to a driving system for electroluminescence displays and the method of operating the system with reduced power demand and increased performance in response speed.
2. Description of Related Art
A typical electroluminescence display is an apparatus based on a panel of electroluminescence elements organized in a two-dimensional matrix of rows and columns. In general, each of the electroluminescence elements of the display has two electrodes of the opposite electric polarity: an anode and a cathode. One of the electrodes is connected to a row line while the other to a column line of the driving circuitry of the system. Each of the electroluminescence elements in the matrix is located where the addressing row and column lines for that particular element intersect.
An electroluminescence element emits light when it conducts electric current. This is achievable when a voltage across the anode and cathode of the element is supplied in the forward polarityxe2x80x94a positive voltage to the anode and a negative to the cathode. Intensity of the light emitted is determined by the magnitude of the current, which, in turn, is dependent on the voltage applied across the electrodes.
In operation, a driving scheme is employed to display data on the panel of the two-dimensional matrix of an electroluminescence display. A typical driving scheme for driving such an electroluminescence display involves sequentially activating each and every of either rows or columns of the electroluminescence elements in the matrix, one at a time in a scanning manner. While each row or column is activated, selected elements in the activated row or column are then turned on via established electrical routes to the power source of the drive system so that they can be energized and emit light. The addressed elements are activated sequentially in the repeated scanning cycles at a speed sufficiently fast such that the sequentially emitting elements appear to the human eyes as being lighted simultaneously, allowing for a properly perceived image.
A common driving scheme in such electroluminescence displays is one that scans the rows in the matrix of display elements. Display element rows in the matrix are addressed one after another sequentially. Meanwhile, appropriate power or ground sources drive the element columns so as to activate or deactivate the electroluminescence elements respectively in accordance with the requirement of the image data to be displayed.
FIG. 11 is a schematic diagram illustrating the circuit configuration of a conventional driving system for an electroluminescence display panel. The prior-art panel exemplified here has a matrix of 64 rows by 132 columns of display elements. In the matrix, each element is designated as elements EC,R, wherein the subscript xe2x80x9cCxe2x80x9d identifies the column and xe2x80x9cRxe2x80x9d the row positional designation. In the entire matrix consisting of E1.1-E132.64, anodes of each column of the electroluminescence display elements are electrically connected together and to their respective anode lines A1-A132. In a similar arrangement, cathodes of each row of the elements are connected to their respective cathode lines B1-B64.
As is seen in the drawing, one among the 64 display element rows, namely the top row with elements connected to the cathode line B1 in the depicted prior-art example, is activated by connection to ground via its assigned cathode line scanning switch 51 of the cathode line scanning circuit 1. Meanwhile, all the other elements in cathode lines B2-B64 remain de-activated by connection to power Vcc via their respective cathode line scanning switches 52-564. Note that cathode line scanning circuit 1 is essentially an array of switches that are responsible for connecting the rows of display elements alternatively to power and ground voltages of the system.
Anode line driving circuit 2, essentially an array of switches for connecting the display element columns to the power source, activates selected ones of the display element columns by connecting them to their respectively-assigned ones of the current sources 21-2132. Such connection is achieved by switching control of the anode line driving switches 61-6132. Columns to be de-activated are instead connected to ground through anode line resetting switches 71-7132 in the anode line resetting circuit 3, which is essentially an array of switches used to selectively connect the columns to ground.
Note that switching operation of the cathode line scanning switches 51-564 in the cathode line scanning circuit 1 is basically in a sequential and cyclic repeating manner. By contrast, switching operation of the anode line driving switches 61-6132 in the anode line driving circuit 2 and switches 71-7132 in the anode line resetting circuit 3 are in synchronism in accordance with the column data of the image to be displayed.
For example, in the prior art system of FIG. 11, elements E1.1. and E2.1 are emitting light while other elements are turned off. In order for elements E1.1 and E2.1 to be turned on and emit light, switches 61 and 71 corresponding to the anode line A1 have to be switched in synchronization while their row, the one at cathode line B1, is scanned. The same applies to switches 62 and 72.
Off elements include those uncharged ones such as E3.1 and charged ones such as E3.2. In the drawings, turned-on display elements are represented by the symbol of a light-emitting diode, and turned-off elements are represented by the symbol of a capacitor, with charged elements expressed as symbols of capacitors with shading, and partially-charged and uncharged ones as regular capacitors. The charged and uncharged status of these turned-off display elements depends on the magnitude of the electric potential present across the electrodes of the elements.
In an electroluminescence display, parasitic capacitance inherent in the elements represents a major problem. Because of large capacitive loading on the lines as well as the effect of charge storage, quality of displayed image deteriorates when light emission time duration for any particular element becomes non-uniform in the repeated frame cycles as a result of different image patterns. The phenomenon that off elements are induced to emit slightly due to signal cross-coupling under large-capacitance load switching conditions also degrades the display quality.
At least one prior art, for example, Okuda et al. in U.S. Pat. No. 5,844,368 titled xe2x80x9cDriving system for driving luminous elementsxe2x80x9d disclosed a system that attempted to minimize these problems by forcing the rows and columns to definite supply levels in order to achieve certain reference situation before activation of the elements. In another prior art, Lee in U.S. Pat. No. 4,975,691 titled xe2x80x9cScan inversion symmetric drivexe2x80x9d disclosed a system that reversed the scan sequence of the rows for each frame. The reversed scanning in its alternating sequences of applications of the write voltage to the row electrodes was hoped to cause the average residual dc voltage across each of the display pixel elements to be substantially reduced, so that problems such as latent image caused by the inherent capacitive characteristics of electroluminescence displays may be avoided. However, in these prior attempts, the large panel capacitance are charged and discharged by supplying large switching currents from the power sources. These switching currents increase with panel size and with scan speed. Minimizing these switching currents and their subsequent noise issues have become important particularly for driving systems used in mobile electronic appliances.
It is therefore an object of the invention to provide a driving system and its corresponding driving scheme for electroluminescence displays that employs the principle of electric charge conservation and recycling in order to reduce switching current requirement against the system power source.
In an electroluminescence display of a matrix of electroluminescence elements arrayed in rows and columns, wherein anodes of the electroluminescence elements on each row being electrically connected to a corresponding anode line, and cathodes of the electroluminescence elements on each column being electrically connected to a corresponding cathode line, a driving system for driving said electroluminescence display comprises a row/column control circuit for lighting at least one of said electroluminescence elements by establishing an electrical route across the power source and the ground in a driving scheme. The driving scheme sequentially scans each of the cathode lines while simultaneously drives at least one of the anode lines during each scanning. The control circuit equalizes electric charges in electroluminescence elements in the cathode line being scanned and in an adjoining cathode line to be scanned subsequently by electrically connecting both cathode lines together before the scanning cycles to the adjoining cathode line.